1. Field of the Invention
The present invention relates to an optical element having desired optical properties, a sensor apparatus using the optical element, and a sensing method, in particular, to detection of the concentrations of biological molecules.
2. Description of the Related Art
It has been known that localized surface plasmon resonance (hereinafter abbreviated as LSPR) is induced on a minute metal structure. The resonance conditions of LSPR are determined by the refractive index and dielectric constant of the environment surrounding the metal structure. Accordingly, the change of the dielectric constant of the environment surrounding the metal structure can be detected by measuring the change of the optical spectrum of the metal structure by irradiating the metal structure with light to transmit the metal structure.
LSPR is sensitive to the changes of the refractive index and dielectric constant of the environment surrounding the metal structure. Accordingly, LSPR can be applied to a high-sensitivity refractive index sensor. Additionally, as shown below, when the change of the dielectric constant is caused by a biological reaction, application of this phenomenon to a biosensor enables a high-sensitivity sensing, and such a sensing is expected to be widely applied to medical fields and fields of food and environment.
For example, an antigen-antibody reaction can be detected by causing the antigen-antibody reaction to occur in the environment surrounding a metal structure, and by detecting the thereby caused change of the dielectric constant of the environment.
An article by Richard P. Van Duyne et al. in NANO LETTERS, Vol. 4, No. 6, pp. 1029-1034 (2004) discloses an example utilizing as metal structures minute Ag thin-film microparticle structures formed on a flat and smooth substrate, and the antigen concentration has been measured from the change between the optical spectrum for a state where the antibody is attached to the environment surrounding the structures and the optical spectrum for a state where the antigen is further bonded to the antibody. Additionally, the same principle as described above enables to detect a complex formation between an enzyme and a substrate, and a complementary base pair formation based on hybridization of DNA.
Additionally, Japanese Patent Application Laid-Open No. 2000-356587 discloses an example in which Au microparticles are immobilized on a glass substrate, from the spectrum of the plasmon resonance of the Au microparticles, the refractive index of the environment surrounding the microparticles has been detected.
In common transmission measurements including such conventional techniques as described above, the change of the dielectric constant of the environment surrounding the metal particles caused by an antigen-antibody reaction is detected. FIGS. 3A and 3B illustrate a process for such a detection. Specifically, the detection is conducted by measuring the transmission spectra of the metal particles before and after the antigen-antibody reaction, namely, the LSPR optical spectrum (before reaction) 301 and the LSPR optical spectrum (after reaction) 302. The concentration of the antigen is detected from the change of the resonance conditions of LSPR between before and after the reaction (FIG. 3A).
As illustrated in FIG. 3B, the measurement system for the above-described case is schematically as follows: the surface of metal particles 303 is modified with an antibody 305; the metal particles 303 are supported on a dielectric substrate 304 to form a measurement element 306; and the measurement element 306 is irradiated with irradiation light 307 and the transmitted light 308 is detected to measure the transmission spectrum of the measurement element 306.